The present invention relates to a method and a device for ensuring the braking effect of brake actuators in a vehicle. Many modifications of such methods and devices are known in the related art.
German Patent Application No. 198 46 392 describes a method of drying a vehicular operating brake having a brake disk and brake linings. The response of the operating brake when driving in rain or snow or on wet roads is improved by the methods described in German Patent Application No. 198 46 392, thus ensuring a shorter stopping distance. Information about the degree of wetness is generated first. On the basis of this information, an automatic drying function is activated, applying the brake linings to the brake disks in such a way that there is essentially no noticeable deceleration of the vehicle.
German Patent No. 44 27 170 describes a method of ensuring the braking effect of brakes in vehicles in wet weather. According to this method, a sensor device determines the presence of wetness. If there is wetness, the time when the brakes are not operated is determined. At least when the brake operation-free time thus determined exceeds a time threshold, a braking operation is performed by a controller provided in the vehicle during a certain braking time. During the braking operation, a brake pressure is generated, the resulting braking effect being determined so that the driver will not perceive any deceleration of the vehicle.
The publications cited above contain activation conditions which, when met, result in the brake actuators present in the vehicle being activated in such a way that braking-diminishing films on these brake actuators are removed. If none of these activation conditions are met, the operation of the gas pedal is considered.
Against this background, the object is as follows: Existing methods and equipment for ensuring the braking effect of brake actuators arranged in the vehicle are to be improved to the extent that the driver""s desire for acceleration can be implemented without being affected.
At this point, reference should be made to the publication xe2x80x9cFDRxe2x80x94Die Fahrdynamikregelung von Boschxe2x80x9d [FDRxe2x80x94driving dynamics control by Bosch] in the automotive engineering journal ATZ, vol. 96, no. 11 (1994) pp. 674-689. This publication describes a device for controlling the yaw rate of the vehicle. To do so, the measured yaw rate is compared with a setpoint for the yaw rate. This comparison determines a system deviation of the yaw rate as a function of which braking measures and/or engine measures are implemented on individual wheels independently of the driver. A yaw moment is applied to the vehicle mainly as a result of the braking measures independent of the driver, causing the actual yaw rate to approach the setpoint.
Furthermore, reference is made to SAE Paper 870337 xe2x80x9cASR traction controlxe2x80x94a logical extension of ABS.xe2x80x9d This SAE paper describes a slip-regulated brake system with which traction control, in addition to brake slip control, can be implemented. Braking measures independent of the driver are also necessary to implement traction control.
The term xe2x80x9cbraking measures independent of the driverxe2x80x9d as used above means that a brake pressure can be generated in the wheel brake cylinders with braking measures independently of the driver even if the driver has not operated the brake pedal. With these braking measures it is also possible to produce a higher brake pressure in the wheel brake cylinders than the admission pressure which is determined by operation of the brake pedal by the driver. To implement braking measures independently of the driver, there must be a pump in the brake system where a brake pressure can be produced independently of the driver.
The content of the publication cited above publication xe2x80x9cFDRxe2x80x94Die Fahrdynamikregelung von Boschxe2x80x9d [FDRxe2x80x94driving dynamics control by Bosch] as well as the content of SAE Paper 870337 are hereby incorporated by reference in the present application.
The present method relates to a method of ensuring the braking effect of brake actuators in a vehicle, each brake actuator containing first and second components for producing the braking effect. With the present method, films on the first components that could interfere with the deployment of the braking effect are removed. To do so, when an activation condition is met, the brake actuators for removing the brake films are controlled in such a way that the second components are applied to the first components, so that there is little or no slowing of the vehicle that could be perceived by the driver.
According to the present invention, a quantity describing the operation of the accelerator pedal is taken into account in the present method in checking on whether the activation condition has been met. The activation condition has been met if the quantity describing the operation of the accelerator pedal indicates that the accelerator pedal has not been operated. This measure ensures that the driver""s desire for acceleration is implemented without being affected. In other words, If the accelerator pedal has been operated, i.e., if the driver intends to accelerate, then in this situation the brake actuators are not activated to remove the films. Consequently, the driver""s intent to accelerate can be implemented without being affected.
In addition to the quantity describing the operation of the accelerator pedal, additional quantities are also analyzed as part of the activation condition. First, quantities related to operation of the brake actuators are analyzed. One of these quantities describes the operation of the brake pedal, i.e., whether the driver is operating the brake pedal and thus braking measures depending on the driver are being implemented.
Another of these quantities represents operation of the brake actuators independently of the driver. In other words, this quantity indicates whether braking measures are being implemented independently of the driver, as is the case, for example, as part of a yaw rate regulator described in the publication cited above xe2x80x9cFDRxe2x80x94Die Fahrdynamikregelung von Boschxe2x80x9d [FDRxe2x80x94driving dynamics control by Bosch] or as part of the traction control described in SAE Paper 870337 cited above. Therefore, the two quantities indicated above are analyzed as part of the activation condition, because it is not necessary to remove the films at the moment when the brake actuators are being operated, whether by the driver or independently of the driver.
Furthermore, a velocity quantity describing the velocity of the vehicle is analyzed. This is done for the following reason. At low velocities, it is much less problematic if the full braking effect is not available from the beginning than at high velocities.
In addition, a quantity representing the operation of the windshield wiper or a sensor for detecting wetness is analyzed. This is done because it is not usually necessary to remove the films until there is wetness.
In summary, the activation condition is met and thus it is necessary to remove the films when:
the quantity describing operation of the accelerator pedal indicates that the accelerator pedal is not being operated, and/or
a quantity describing operation of the brake pedal indicates that the brake pedal is not being operated, and/or
a velocity quantity describing the velocity of the vehicle is greater than a respective threshold value, and/or
a quantity representing operation of the brake actuators independently of the driver indicates that there is operation of the brake actuators independently of the driver, and/or
a quantity representing operation of the windshield wiper indicates that the windshield wiper is being operated, or if a sensor for detecting wetness indicates that there is wetness.
The activation condition is usually met when all the individual conditions listed above are met at the same time. However, this should not be a restriction. It is also conceivable for the activation condition to be met even if only some of these individual conditions are met. For example, the windshield wiper need not necessarily be in operation or a sensor for detecting wetness need not be indicating the presence of wetness. This individual condition may also be disregarded. Finally, it is also conceivable for not only the films caused by wetness but also films or deposits in general to be removed by the method according to the present invention. In this case, removal of films would not be possible under dry road conditions if this individual condition were retained.
It has proven advantageous that a temperature quantity describing the outside temperature is analyzed as part of the activation condition. In this case, the activation condition is met when the temperature quantity is also lower than a predetermined temperature value. The outside temperature is taken into account as part of the activation condition, because driving on road surfaces in winter or when wet and salty represents a special problem. Driving on a road in winter or when wet and salty can be detected by simultaneous analysis of the outside temperature and operation of the windshield wiper or a sensor for detecting wetness. If the method according to the present invention is used exclusively for removing films due to driving on road surfaces in winter or when wet and salty, in this case, from the standpoint of the activation condition, it would be sufficient to analyze the individual conditions with respect to the outside temperature and operation of the windshield wiper or the sensor for detecting wetness.
It has proven advantageous if to remove the films the brake actuators are not activated so that a predetermined brake pressure is set directly by the control action, but instead, the brake actuators are activated as part of a vehicle control system.
To be able to set the brake pressure at a predetermined level, i.e., to regulate the brake pressure, the actual prevailing pressure must be available so that the actual pressure can be made to approach the setpoint pressure through the control operation. However, this poses a problem. Although slip-controlled brake systems such as those described in the ATZ article or SAE paper cited above usually have means with which the actual pressure can be estimated, the pressure values thus estimated cannot be used for the present application.
The brake pressures to be set according to the present invention to remove the films are on the order of magnitude of approximately five bar. In this pressure range, the estimate of the actual pressure is not very accurate. Consequently, this can result in a buildup of a higher brake pressure through brake pressure control than actually desired, because, for example, there is no system deviation, although one is indicated. Ultimately, this excessively high brake pressure which has been set incorrectly would result in the driver perceiving the removal of the films because of the resulting slowing of the vehicle.
However, removal of the films should be a comfort function which should take place unnoticed by the driver. Against this background, as mentioned above, a brake pressure should not be directly set by preselecting a setpoint pressure, i.e., regulating the brake pressure. Preselection of a setpoint pressure is therefore critical. In the case of an incorrect estimate, not only may the brake pressure be set too high, but there may not be any pressure buildup at all.
It has proven advantageous to regulate not the brake pressure but instead the deceleration of the vehicle as a means of removing the films. In other words, the brake pressure necessary to remove the films is not set directly, but instead is set indirectly by regulating the deceleration of the vehicle. Deceleration of a vehicle is generally regulated as follows. A setpoint for the vehicle deceleration is selected. A system deviation is determined by comparing the actual value determined for the vehicle deceleration with the setpoint. The brake actuators are controlled as a function of this system deviation so that the actual value of the vehicle deceleration approaches the setpoint. If the absolute value of the setpoint is greater than that of the actual value, the brake actuators are controlled so that brake pressure is supplied to the wheel brake cylinder. However, if the absolute value of the setpoint is lower than the absolute value of the actual value, the brake actuators are controlled so that brake pressure in the wheel brake cylinders is reduced. Controlling vehicle deceleration has the advantage that this control is very accurate, because the actual value for the vehicle deceleration can be determined very accurately as a function of the wheel speeds determined.
The activation condition indicates that removal of the films is necessary, and therefore the brake actuators must be activated as part of controlling vehicle deceleration, so the setpoint for the vehicle deceleration is determined in an advantageous manner as soon as the activation condition is met. Thus, as soon as the activation condition has been met, vehicle deceleration control can be implemented. As indicated above, vehicle deceleration is controlled at least by operation of the brake actuators as a function of the setpoint. Due to this operation of the brake actuators, the films on the first components of the brake actuators are removed. To summarize, the present method ensures that sufficient pressure is created in the wheels and that the driver will hardly notice any impairment at all in driving performance.
As mentioned above, one option is to determine a deceleration quantity which describes the deceleration of the vehicle and is necessary to control the vehicle deceleration, as a function of wheel speed quantities. This is suggested because the above-mentioned slip-controlled brake systems have wheel rpm sensors assigned to the vehicle wheels, i.e., no additional sensors are necessary.
As mentioned above, operation of the accelerator pedal is analyzed as part of the activation condition, i.e., the activation condition is met if there is at least no operation of the accelerator pedal. First, this ensures, as mentioned previously, that the driver""s intent to accelerate is implemented without being hindered, because the films are not removed if the accelerator pedal is being operated.
However, there is also a second reason for taking into account the operation of the accelerator pedal. The activation condition is met if there is at least no operation of the accelerator pedal. In other words, in order for the activation condition to be met, the driver must have let off on the accelerator pedal. Since the accelerator pedal has been released, the engine no longer delivers any driving torque. Instead of this, it produces a drag moment which results in deceleration of the vehicle. Since the setpoint for the vehicle deceleration is determined as soon as the activation condition is met, i.e., at the start of the deceleration process initiated on the basis of the drag moment produced by the engine, the deceleration induced when the driver releases the accelerator pedal is used to remove the films, i.e., it is not necessary to xe2x80x9cartificiallyxe2x80x9d induce deceleration which the driver might perceive as uncomfortable. Consequently, the setpoint for vehicle deceleration is determined in an advantageous manner as a function of the value of the deceleration quantity prevailing at the time when the activation condition is met. However, to implement the operation of the brake actuators necessary for removing the films, an offset must also be taken into account in determining the setpoint.
This should take place in such a form that the absolute value of the setpoint is greater than the absolute amount of the value of the deceleration quantity prevailing at the time when the activation condition is met. If the setpoint is defined as a positive quantity, then the offset is also positive. However, if the setpoint is defined as a negative quantity, the offset is also negative.
To summarize, the setpoint deceleration is supplied at the beginning of the activation and is formed as a function of the instantaneous vehicle deceleration at that time plus an extra deceleration factor. Since slip-controlled brake systems have wheel rpm sensors, the setpoint deceleration is derived directly from measured quantities and thus can be determined reliably. The vehicle deceleration is influenced in a defined manner by preselecting a setpoint deceleration and thus can be controlled precisely. The desired brake pressure buildup which is necessary for removal of the films takes place according to the extra deceleration factor. The offset can be supplied in various ways. The offset is advantageously a fixedly predetermined value which is applied in advance via analysis of preliminary driving trials. However, the offset may also be a value obtained with a predetermined factor by weighting the value of the deceleration quantity prevailing at the time when the activation condition is met. This predetermined factor is also applied in advance in preliminary driving trials.
The following procedure has proven advantageous in checking the activation condition. The activation condition is composed of individual conditions, as mentioned previously. For successive points in time, at least one individual condition is checked. In other words, a check is performed for each of the successive points in time to determine whether at least one of the individual conditions is met at this time. There is an essentially uniform predetermined interval between the successive points in time, i.e., the successive points in time form a time grid having predetermined intervals. This interval is applied in advance through preliminary driving trials. Computing capacity of the controller is not tied up unnecessarily because of this analysis of the activation condition taking place in a time grid.
The interval of the time grid is selected so that removal of the films can be performed if necessary, i.e., if experience has shown that a film has again formed on the first components of the brake actuators. The determination of the interval is based on experience obtained through driving trials. The interval is preferably on the order of five to ten minutes.
The procedure of checking whether the activation conditions have been met at certain points in time can be interpreted as an activation request being supplied at certain points in time and different individual conditions are checked if this activation request is present. The activation condition is met as soon as at least one of these individual conditions is met when there is an activation request.
The successive points in time normally have a predetermined interval between them. However, this interval could be influenced in two cases. First, the sequence of the successive points in time and thus the activation request can be reset by successful removal of the films or by operation of the brake actuators either by the driver or independently of the driver. This means that as soon as the films have been removed or after operation of the brake actuators either by the driver or independently of the driver, the next point in time follows directly. This point in time thus does not have the predetermined interval from the directly preceding point in time. The time grid is more or less shifted. The following points in time again have the predetermined interval.
This takes place against the background that after removal of the films or after operation of the brake actuators either by the driver or independently of the driver, the films are removed and a new deceleration of the first components is necessary again only after the predetermined interval.
The procedure described above can be summarized as follows. If the brake actuators are operated either by the driver or independently of the driver or if the brake actuators are activated, in particular to remove the films, between two successive points in time, then at least the next point in time is selected as a function of this operation or activation. It is provided in particular here that the following points in time are based on this operation or activation.
In an advantageous embodiment, when the brake actuators are operated either by the driver or independently of the driver or when the brake actuators are activated between two successive points in time, in particular to remove the films, the next point in time follows directly or indirectly after successful operation or activation and the following points in time are based on this operation or activation.
In addition, the preset interval may be adapted to the prevailing boundary conditions. In the present case, for example, these should be defined by the outside temperature and/or the intensity of the wetness. The preset interval is advantageously modified as a function of a temperature quantity describing the outside temperature in such a way that this interval is smaller at lower temperatures than at high temperatures. In addition, the preset interval is modified as a function of a quantity describing operation of the windshield wiper in such a way that the interval becomes smaller with increasing windshield wiper speed. As an alternative or in addition to the analysis of operation of the windshield wiper, a rain sensor which is optionally present in the vehicle may be analyzed. In general form, a sensor for detecting wetness on the road surface may be analyzed.
If the activation condition is met, the brake actuators are generally activated on all wheels of the vehicle to remove the films. This activation of the brake actuators may also take place only for some of the vehicle wheels to advantage. Since the braking effect on the front axle is greater than that on the rear axle in a braking operation, it is advantageous if at least the brake actuators on the front axle of the vehicle are operated, so at least the films are removed from the wheels on the front axle.
Activation of the brake actuators to remove the films on the first components is terminated when a deactivation condition is met. Various individual conditions are analyzed as part of the deactivation condition. For example, the period of time during which the brake actuators are controlled to remove the films and/or operation of the accelerator pedal and/or operation of the brake pedal and/or the velocity quantity and/or a quantity representing operation of the brake actuators independently of the driver is analyzed.
The deactivation condition is met when:
the period of time during which the brake actuators are activated to remove the films is greater than a preset time value, and/or
the quantity describing operation of the accelerator pedal indicates that the accelerator pedal is being operated, and/or
a quantity describing operation of the brake pedal indicates that the brake pedal is being operated, and/or
a velocity quantity describing the velocity of the vehicle is below a respective threshold value, and/or
a quantity representing operation of the brake actuators independently of the driver indicates that the brake actuators are being operated independently of the driver.
The brake actuators may be designed as disk brakes or as drum brakes. If the brake actuators are disk brakes, they have brake disks as the first components and brake linings as the second components. If the brake actuators are drum brakes, they have a brake drum as the first component and brake shoes as the second component.
With the method according to the present invention, the operability of the wheel brakes should be maintained even under unfavorable conditions, e.g., in winter, on icy roads or when driving for a long time without operation of the brakes.